SCHED_4BSD is the old, tried and true, original CPU scheduler. Works well on single-CPU systems. Doesn't scale to multi-CPU setups all that well. Especially on systems with 4 or more CPUs. This is the defaut scheduler on FreeBSD 4.x, 5.x, 6.x, and 7.0.

SCHED_ULE is the new, optimised-for-SMP scheduler that will eventually become the default scheduler for 7.x and beyond. Under some workloads, it's also better than 4BSD on single-CPU systems.

On FreeBSD 7.0, it's still considered experimental, but is recommended for those with multi-CPU or multi-core systems. A lot of fixes have gone into 7-STABLE (tag=RELENG_7 in csup supfiles), and it will be the default scheduler in 7.1.